High-performance supercapacitors based on metal nanowire yarns

ABSTRACT

An energy-storage device is formed from a first and a second yarn, each yarn including a plurality of nanowires including aluminum and/or a transition metal. An anode pad is in contact with the first yarn and a cathode pad is in contact with the second yarn. Alternatively, first and second metallic electrodes may be disposed substantially in parallel, with pluralities of nanowires including aluminum and/or a transition metal extending therefrom. In another embodiment, a supercapacitor may include a niobium yarn including a plurality of niobium nanowires. Each niobium nanowire may include at least (i) a first section comprising at least one of unoxidized niobium and niobium oxide; (ii) a second section comprises a niobium pentoxide layer; and (iii) a third section comprises a layer formed by dipping the niobium nanowire in at least one of a conductive polymer and a liquid metal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/839,499, filed Aug. 28, 2015, which claims priority to and thebenefit of, U.S. Provisional Patent Application No. 62/043,490, entitled“Flexible, Sewable, and Tunable Energy Storage Device,” which was filedon Aug. 29, 2014; both applications are hereby incorporated by referencein their entireties.

FIELD OF THE INVENTION

This disclosure generally relates to energy storage devices and, inparticular, to supercapacitors made using yarns of transition-metalnanowires.

BACKGROUND

Conventional capacitors such as parallel-plate capacitors andelectrolytic capacitors typically have low capacitance values in therange of 1 pF to about 1 mF. These capacitors also have relatively lowenergy density of, e.g., about 70 mWKg⁻¹, making them generallyunsuitable for use in wearable computing, e.g., via integration of thesecapacitors with clothing and apparel. In batteries, electrode surfaceand bulk material of the battery are involved in the charge storagemechanism, which generally increases the energy density of batteriesrelative to the energy density of conventional capacitors. The powerdensity of a battery, however, is relatively low because of the chargestorage mechanism of batteries is relatively slow compared to the chargestorage mechanism of a conventional capacitor. Specifically, in contrastto batteries, charge storage in capacitors occurs at the surface, eitherin the electric double layer or redox states, and the bulk material ofthe capacitor does not contribute significantly to the charge storagewhich can make the charge storage faster compared to batteries. Whilethis makes the energy density of a capacitor lower compared to that ofbatteries, the power density is relatively greater.

Carbon nanotube (CNT) and/or graphene based energy storage devices(generally called supercapacitors) typically have a high gravimetriccapacitance of, e.g., about 120 F/g. Miniature energy storage devicesmade using CNT and/or graphene in forms such as nanotubes, yarns,forests, etc., can therefore offer both high energy densities and highpower densities. The large ion accessible surface area of CNTs andgraphene sheets formed as yarns, forests, and films can enable miniaturehigh performance supercapacitors with power densities exceeding those ofelectrolytics, while achieving energy densities comparable to those ofbatteries. Capacitance and energy density can be enhanced by depositinghighly pseudocapacitive materials such as conductive polymers. Yarnsformed from carbon nanotubes were proposed for use in wearablesupercapacitors.

These CNT based yarns, however, typically have low tensile strengthe.g., of about 0.6-1.1 GPa, and CNT is generally not biocompatible.Therefore, CNT yarn based supercapacitors are not particularly wellsuited for making wearable supercapacitors and for integration withclothing and apparel. CNTs also typically do not withstand hightemperatures, e.g., temperatures above 400-500° C., or above 2000° C. invacuum. Therefore, many CNT-based capacitors are also not suitable foroperation in high-temperature environments, where the temperature canexceed 1000° C., such as in various sensors used in drilling for oil.

SUMMARY

Various supercapacitor structures described herein have high volumetriccapacitance and high tensile strength. These supercapacitor structurestypically have a relatively low resistance, or high conductivityrelative to carbon-based devices, and they are generally biocompatible.This is achieved, at least in part, by constructing supercapacitorstructures using yarns of nanowires of transition and other metals suchas niobium, tantalum, vanadium, molybdenum, copper, nickel, iron,platinum, gold, silver, zinc, and aluminum. Additional optionalmaterials such as separators, electrolytes, metallic electrodes, etc.,are used in various supercapacitor structures to enhance propertiesthereof, such as volumetric capacitance, low surface resistance, etc.

Accordingly, in one aspect a supercapacitor includes a first yarn and asecond yarn. Each yarn includes several nanowires that include aluminumand/or a transition metal. An anode pad is in contact with the firstyarn, and a cathode pad is in contact with the second yarn. Thetransition metal can be any one of or a combination of two or more ofniobium, tantalum, vanadium, molybdenum, copper, nickel, iron, platinum,gold, silver, and zinc.

The first and second yarns may include the same material, i.e., aluminumand/or a transition metal. In other embodiments, the first and secondyarns may include different materials.

In some embodiments, the diameter of each nanowire is selected from arange of 20 nm to 200 nm, and a length of the first yarn is selectedfrom a range of 1 μm to 100 m. The diameter of the first yarn may beselected from a range of 10 μm to 1 mm.

In some embodiments, each yarn is coated with a flexible, solidelectrolyte. The flexible, solid electrolyte may include polyvinylalcohol (PVA) and sulfuric acid. In general, the flexible, solidelectrolyte may be formed using an ionic liquid and inert nanoparticlessuch as fused silica nanoparticles. The first yarn and the second yarnmay be twisted together forming a twisted pair of yarns. The capacitanceof the supercapacitor may vary substantially linearly according to alength of the twisted pair of yarns.

In some embodiments, the first or the second yarn, or both yarns may becoated with a pseudocapacitive material. The pseudocapacitive materialmay include a conductive polymer, and the conductive polymer may includeone or more of poly(3,4-ethylenedioxythiophene) (PEDOT), poly pyrrole,and poly aniline.

In some embodiments, the supercapacitor of claim 1, further includes asealed enclosure encapsulating the first and second yarns. The sealedenclosure may include a liquid electrolyte and an ionically conductiveseparator disposed within the sealed enclosure and between the first andsecond yarns. The liquid electrolyte can be an aqueous electrolyte, anorganic electrolyte, an ionic electrolyte, or a molten salt. In someembodiments, the liquid electrolyte is sulfuric acid, tetrabutylammoniumhexafluorophosphate (TBAPF6) in acetonitrile, or tetraethylammoniumtetrafluoroborate in propylene carbonate. The separator may include oneor more of: (i) glass fibers, (ii) a perfluorosulfonic acid polymer,(iii) a millipore membrane, and (iv) a cellulosic-based sheet. Thecellulosic-based sheet may include micron-sized cellulosic wood pulpfibers.

In some embodiments, the volumetric capacitance of the supercapacitor isat least 5 F/cm³. The operating voltage of the supercapacitor may be atleast 2.5 V, and the total capacitance of the supercapacitor may be atleast 10 mF. In some embodiments, the conductivity of the supercapacitoris at least 3×10⁴ S/m. The energy density of the supercapacitor may beat least 5 MJ/m³. The peak power density of the supercapacitor may be atleast 22 MW/m³.

In another aspect, a supercapacitor includes a first metallic electrodeincluding a metal, and a first group of nanowires including aluminumand/or a transition metal. The first group of nanowires extends from thefirst metallic electrode. The supercapacitor also include a secondmetallic electrode including the metal and disposed substantially inparallel to the first metallic electrode. A second group of nanowiresalso includes aluminum and/or the transition metal and extends from thesecond metallic electrode. The transition metal can be any one of or acombination of two or more of niobium, tantalum, vanadium, molybdenum,copper, nickel, iron, platinum, gold, silver, and zinc. The firstmetallic electrode may include one or more of gold, platinum, silver,copper, and aluminum.

In some embodiments, the first and second metallic electrodes mayinclude the same metal. In other embodiments, the first and secondmetallic electrodes may include different metals. The first and secondgroups of nanowires may include the same material, i.e., aluminum and/ora transition metal. In other embodiments, the first and second groups ofnanowires include different materials.

In some embodiments, the supercapacitor further includes an electrolyte,and the metal and the electrolyte are selected to be compatible witheach other. The area of the first metallic electrode may be selectedfrom a range of 6 mm² to 600 mm². The diameter of each nanowire may beselected from a range of 20 nm to 200 nm, and the length of eachnanowire may selected from a range of 1 μm to 1,000 μm. In someembodiments, between 10% and 40% volumetric space of the first metallicelectrode includes or is occupied by the first group of nanowires.

The first, second, or both groups of nanowires may be coated with apseudocapacitive material. The pseudocapacitive material may include aconductive polymer, and the conductive polymer may include one or moreof poly(3,4-ethylenedioxythiophene) (PEDOT), poly pyrrole, and polyaniline. In some embodiments, the supercapacitor further includes anionically conductive separator disposed between the first group ofnanowires and the second group of nanowires. The ionically conductiveseparator may include one or more of: (i) glass fibers, (ii) aperfluorosulfonic acid polymer, (iii) a millipore membrane, and (iv) oneor more cellulosic-based sheets. The cellulosic-based sheet or sheetsmay include micron-sized cellulosic wood pulp fibers. In someembodiments, the resistance of the first group of nanowires is less than1.5×10⁻⁴ ohms.

In another aspect, a supercapacitor includes a niobium yarn includingseveral niobium nanowires. Each niobium nanowire includes at least threesections. The first section includes unoxidized niobium, niobium oxide,or both. The second section includes a niobium pentoxide layer, and thethird section includes a layer formed by dipping the niobium nanowire,after the niobium pentoxide layer is formed, in a conductive polymer, aliquid metal, or a combination thereof. The liquid metal may include oneor more of indium, gallium, and tin.

In some embodiments, the diameter of the niobium nanowire is selectedfrom a range of 20 nm to 200 nm, and the thickness of the second sectionis selected from a range of 7.5 nm up to 95 nm. The diameter of theniobium nanowire and the thickness of the second section may be selectedaccording to a specified operating voltage.

In another aspect, a method of constructing a supercapacitor includesthe steps of selecting and grouping together a first collection or setof nanowires including aluminum and/or a transition metal, to form afirst yarn. The method also includes selecting and grouping a secondcollection or set of nanowires including aluminum and/or the transitionmetal, to form a second yarn. In addition, the method includes formingan anode pad in contact with the first yarn, and forming a cathode padin contact with the second yarn. The transition metal can be any one ofor a combination of two or more of niobium, tantalum, vanadium,molybdenum, copper, nickel, iron, platinum, gold, silver, and zinc.

In some embodiments, the nanowires of the first yarn includes the samematerial as the nanowires of the second yarn. In other embodiments, thenanowires of the first and second yarns include different materials.

In some embodiments, selecting the first set of nanowires includesselecting a diameter of each nanowire from a range of 20 nm to 200 nm,and selecting a length of the nanowires from a range of 1 μm to 100 m.Grouping the first set of nanowires into the first yarn may includeselecting a diameter of the first yarn from a range of 10 μm to 1 mm.

In some embodiments, the method also includes coating each yarn with aflexible, solid electrolyte. The method may include forming theflexible, solid electrolyte using PVA and sulfuric acid. The flexible,solid electrolyte may also be formed using an ionic liquid and inertnanoparticles. The method may further include twisting the first yarnand the second yarn together to form a twisted pair of yarns. In someembodiments, the method additionally includes coating the first yarn,the second yarn, or both yarns with a pseudocapacitive material. Thepseudocapacitive material may include a conductive polymer, and theconductive polymer can be one or more ofpoly(3,4-ethylenedioxythiophene) (PEDOT), poly pyrrole, and polyaniline.

In some embodiments, the method includes disposing the first and secondyarns within an enclosure, and disposing an ionically conductiveseparator within the enclosure and between the first and second yarns.The method may also include disposing a liquid electrolyte within theenclosure, and sealing the enclosure. The liquid electrolyte can be anaqueous electrolyte, an organic electrolyte, an ionic electrolyte, or amolten salt. In some embodiments, the liquid electrolyte can be sulfuricacid, tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile,or tetraethylammonium tetrafluoroborate in propylene carbonate. Theseparator may include one or more of: (i) glass fibers, (ii) aperfluorosulfonic acid polymer, (iii) a millipore membrane, and (iv) oneor more cellulosic-based sheets. The cellulosic-based sheet or sheetsmay include micron-sized cellulosic wood pulp fibers.

In another aspect, a method of constructing a supercapacitor includesthe steps of forming a first metallic electrode having a metal on afirst group of nanowires including aluminum and/or a transition metal.The method also includes forming a second metallic electrode having themetal on a second group of nanowires including aluminum and/or thetransition metal, and disposing the second metallic electrodesubstantially in parallel to the first metallic electrode. Thetransition metal can be any one of or a combination of two or more ofniobium, tantalum, vanadium, molybdenum, copper, nickel, iron, platinum,gold, silver, and zinc. The first and second metallic electrodes mayinclude one or more of gold, platinum, silver, copper, and aluminum.

In some embodiments, the first and second metallic electrodes mayinclude the same metal. In other embodiments, the first and secondmetallic electrodes may include different metals. The first and secondgroups of nanowires may include the same material, i.e., aluminum and/ora transition metal. In other embodiments, the first and second groups ofnanowires include different materials.

In some embodiments, the method further includes disposing anelectrolyte on the first group of nanowires, and selecting the metal ofthe first metallic electrode and the electrolyte such that the metal andthe electrolyte are compatible with each other. Forming the firstmetallic electrode on the first group of nanowires may includeelectrochemical plating. In some embodiments, forming the first metallicelectrode on the first group of nanowires includes consuming between 10%and 40% volumetric space of the first metallic electrode by the firstgroup of nanowires.

The method may further include coating the first, second, or both groupsof nanowires with a pseudocapacitive material. The pseudocapacitivematerial may include a conductive polymer, and the conductive polymermay include one or more of poly(3,4-ethylenedioxythiophene) (PEDOT),poly pyrrole, and poly aniline. In some embodiments, the method furtherincludes disposing an ionically conductive separator between the firstgroup of nanowires and the second group of nanowires. The ionicallyconductive separator may include one or more of: (i) glass fibers, (ii)a perfluorosulfonic acid polymer, (iii) a millipore membrane, and (iv) acellulosic-based sheet or sheet. The cellulosic-based sheet or sheetsmay include micron-sized cellulosic wood pulp fibers.

In another aspect, a method of constructing a supercapacitor includesthe steps of, for each one of several niobium nanowires, oxidizing aportion of a niobium nanowire, thereby forming a dielectric layer ofniobium pentoxide disposed on an anode portion of the niobium nanowire.The anode portion may include unoxidized niobium, and the method mayinclude optionally oxidizing at least a part of the anode portion,forming a layer of niobium oxide which is separate from the dielectriclayer of niobium pentoxide. In addition, the method includes coatingeach niobium wire, having the anode and the dielectric layer, with oneor more of a conductive polymer and a liquid metal, thereby forming acathode. The method further includes forming a niobium yarn by groupingtogether the niobium wires, each wire having an anode, a dielectriclayer, and a cathode, to form the supercapacitor including the niobiumyarn.

The liquid metal may include one or more of indium, gallium, and tin. Insome embodiments, a diameter of each niobium nanowire is selected from arange of 20 nm to 250 nm, and (ii) oxidizing is controlled such that athickness of the dielectric layer is within a range of 7.5 nm up to 100nm. The method may further include selecting a diameter of each niobiumnanowire and a thickness of each dielectric layer to facilitateoperation of the supercapacitor at a specified operating voltage.

The gravimetric capacitance of CNT and/or graphene based energy storagedevices is generally higher than that of devices based on aluminumand/or transition metals. As such, devices made using aluminum and/ortransition metal yarns may be less preferable than CNT/graphene baseddevices when light-weight energy storage devices are needed. The use ofyarns of aluminum or a transition metal, however, allows many of thesupercapacitor structures described herein to be flexible, and sewable,because the tensile strength of these supercapacitors is generallygreater than that of CNT/graphene based devices. Moreover, thevolumetric capacitance of aluminum or transition-metal-yarn basedsupercapacitors is generally greater than that of CNT/graphene basedsupercapacitors. The aluminum or transition-metal-yarn basedsupercapacitors also have low resistance relative to CNT orgraphene-based devices having a similar capacitance. Therefore, thevarious supercapacitors described herein can be integrated with clothingand apparel, and can be generally used in wearable computing. Moreover,these supercapacitor structures can also withstand temperatures of up toabout 2600° C. if encapsulation of the supercapacitor substantiallyprevents oxidation thereof and, as such, they can be employed inhigh-temperature environments.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIGS. 1A-1C show scanning electron microscopy (SEM) images obtainedduring the formation of a niobium nanowire yarn, according to oneembodiment;

FIG. 1D shows a capacitance-voltage (CV) plot of the yarns depicted inFIGS. 1B and 1C;

FIGS. 1E and 1F are SEM images of separators that may be used insupercapacitors according to different embodiments;

FIG. 2 schematically depicts a charge storage mechanism of an Nb NW yarnbased supercapacitor cell;

FIG. 3 shows energy-dispersive X-ray spectroscopy (EDX) of an Nb NW yarnaccording to one embodiment;

FIGS. 4A-4D are SEM images of niobium nanowires, according to oneembodiment;

FIGS. 4E and 4F are TEM images corresponding to the nanowires shown inFIGS. 4A-4D;

FIG. 5 shows a relationship between gravimetric capacitance and mass ofan Nb NW yarn, according to one embodiment.

FIG. 6 shows CV curves illustrating the effect of gold wrapping on an NbNW yarn, according to one embodiment;

FIG. 7A shows the volumetric capacitance as a function of scan rate ofan Nb NW yarn-based supercapacitor, according to one embodiment;

FIGS. 7B-7E show charging and discharging characteristics of a bare NbNW yarn-based supercapacitor and a PEDOT coated Nb NW yarn-basedsupercapacitor, according to different embodiments;

FIGS. 8A-8C are SEM images of three different microfiber (MF)separators, according to three embodiments;

FIG. 9 shows the volumetric capacitance of an exemplary Nb NW based yarnas a function of the diameter thereof;

FIG. 10 shows a relationship between capacitance and inserted twist ofan exemplary Nb NW yarn;

FIGS. 11A and 11B show twisted Nb NW based supercapacitors according tovarious embodiments;

FIGS. 12A-12C show CV curves for Nb NW-based supercapacitors, accordingto various embodiments;

FIGS. 13A-13D show life cycle tests of Nb NW yarn-based supercapacitorsaccording to different embodiments;

FIG. 13E shows a relationship between capacitance and length of an Nb NWyarn-based supercapacitor having a solid electrolyte, according to oneembodiment;

FIG. 14 depicts a circuit model of an Nb NW yarn based supercapacitor,according to one embodiment;

FIGS. 15A-15B show impedance of an exemplary Nb NW yarn-basedsupercapacitor and a corresponding circuit model;

FIG. 16 shows a comparison of Equivalent Series Resistance (ESR) forbare and PVA coated Nb NW based supercapacitors, according to twoembodiments;

FIG. 17 shows Coulombic efficiency of a supercapacitor, according to oneembodiment;

FIG. 18 shows a comparison of Ragone plots for a lithium-based battery,a carbon fiber supercapacitor, a Maxwell supercapacitor, and Nb NW yarnbased supercapacitors, according to various embodiments;

FIG. 19 shows CV curves at different scan rates of a bare Nb NW yarnbased supercapacitor cell, according to one embodiment;

FIG. 20 shows CV curves at different scan rates of a PEDOT coated Nb NWyarn based supercapacitor cell, according to one embodiment;

FIG. 21 schematically depicts an Nb NW yarn-based supercapacitorstructure, according to one embodiment;

FIG. 22 schematically depicts an Nb NW yarn-based supercapacitorstructure with a liquid electrolyte, according to one embodiment;

FIG. 23A-23D schematically depict an Nb NW yarn-based supercapacitorhaving two separators, according to one embodiment;

FIG. 24 depicts a niobium nanowire based supercapacitor having metallicelectrodes; according to one embodiment; and

FIG. 25 shows an Nb NW yarn-based supercapacitor having a niobiumpentoxide layer, according to one embodiment.

DETAILED DESCRIPTION

In various embodiments described below, niobium nanowires (Nb NW)generally represent nanowires including aluminum and/or varioustransition metals such as tantalum, vanadium, molybdenum, copper,nickel, iron, platinum, gold, silver, and zinc. Niobium is used as anexample for the sake of convenience. It should be understood that thetechniques, structures, and manufacturing processes described herein aregenerally applicable to supercapacitor structures based on nanowires ofother transition metals.

Lack of biocompatibility of carbon nanoparticles limits theirapplication in biomedical devices and implants. Niobium, which is moreabundant than molybdenum, silver and tin, and widely used in steelalloys, is a highly chemically stable, hypoallergenic, biocompatible,and bioinert material, which makes it appropriate for applications injewelry, biomedical, and corrosive resistant coatings for surgicaltools. Volumetric capacitance of bare niobium nanowire yarns is measuredto be three times higher than that of carbon nanotube yarns. Thiscombination of high electrical conductivity and high volumetriccapacitance makes possible the high power and energy densities for thebare niobium nanowire (Nb NW) yarns. Although energy density is lowerthan in some graphene and activated carbon electrodes, these materialscannot achieve the same power density without the use of a metal backinglayer, which would make them suitable for use in wearable devices.

Electrochemical properties of Nb NW yarns were measured and theirperformance as supercapacitor electrodes was evaluated. Electrochemicalcharacteristics of bare Nb NW yarns are reported in variouselectrolytes. A highly ionically conductive cellulose-based separatorwas designed for use in some embodiments of the supercapacitor. To boostthe performance, conducting polymer Poly (3,4-ethylenedioxythiophene)(PEDOT) may be deposited on the electrodes. For the purpose ofillustration, a bare Nb NW based supercapacitor is shown to harvestenergy from a solar cell and then to energize a temperature sensor andan FM transmitter.

Niobium nanowires, depicted in FIGS. 1A and 1B, are twisted into yarnsas shown in FIG. 2, forming high internal surface area materials (e.g.,100 times that of Ni foam). The individual strands Nb NW are about 140nm in diameter. FIG. 1A is an SEM image of a niobium nanowire yarnhaving un-etched copper micro particles that may be removed during thecopper etching process. FIGS. 1B and 1C show SEM images of a generallycopper-free niobium yarn before and after PEDOT deposition,respectively. In FIG. 1D, curve d₁ shows an increase in volumetriccapacitance (with unit of MF·m⁻³ or F·cm⁻³) of one electrode bydepositing PEDOT relative to that of shown by curve d₂ on bare Nbnanowires at scan rate of 500 mV·s⁻¹. The CV was determined from −1 V to1 V demonstrating that the CV is symmetric. The glitches at the zeropotential are due to the artifact of the measurement device. FIG. 1Edepicts thin paper (Kim wipe) as a separator, and FIG. 1F shows highperformance separators prepared by cellulosic wood pulp fibers ofdifferent sizes. Scale bars for all SEM images are 5 μm except for FIG.1E, which is 20 μm. To estimate the specific capacitance expected of theNb NW yarns, the capacitance per area (C_(A)) of bulk niobium wasmeasured, and found to be 0.52 F·m⁻² (52 μF·cm⁻²). Using the value forC_(A) and the 140 nm average diameter of the Nb NW yarns, an estimatedspecific volumetric capacitance limit of 1.36×10⁷ F·m⁻³ (13.6 F·cm⁻³)and gravimetric capacitance limit of 1.5 F·g⁻¹ was found which is lowerthan the gravimetric capacitance of carbon-based materials.

To this end, specific gravimetric and volumetric capacitance limits areestimated from the measured capacitance per area of bulk niobium (0.52F·m⁻²) assuming all the nanowires have circular cross-section:C _(V) ^(o) =C _(A)2/r=1.5×10⁷ F·m⁻³,where r is the average radius of the Nb nanowires (70 nm). Including anestimated packing density of the nanowires the volumetric capacitancecan be found as:

$C_{V} = {{1.5 \times 10^{7}\frac{\pi}{2\sqrt{3}}} \approx {1.36 \times 10^{7}\mspace{14mu}{F \cdot {m^{- 3}.}}}}$

With reference to FIGS. 3 and 4, EDX analysis followed by SEM imagingshows the etched samples are pure niobium with almost no copper traceson them. With reference to FIG. 3 EDX analysis of fully etched niobiumnanowire yarns shows that almost no copper is left on the samples. Thescale bar is 500 nm. The SEM images depicted in FIGS. 4A and 4D weretaken during the etching process. FIG. 4A shows that individual niobiumnanowires are still linked together by copper. FIGS. 4E and 4F are theTEM images of a niobium nanowire. FIG. 4E shows an oxide layer of 4 to 5nm thickness. The diameter of that nanowire is around 180 nm as shown inFIG. 4F. The scale bar for FIGS. 4A, 4B, and 4D is 2 μm; for FIG. 4C is1 μm; for FIG. 4E is 5 nm; and for FIG. 4F is 20 nm. The density of bulkniobium was used to find the gravimetric capacitance of the niobium as:

$C_{m} = {\frac{C_{V}}{\rho} \approx {1.5\mspace{14mu}{{kF} \cdot {{{kg}^{- 1}\left( {F \cdot g^{- 1}} \right)}.}}}}$

Capacitances of the Nb NW yarns were measured in aqueous, organic, andionic liquid electrolytes and are listed in Table 1 below. Inparticular, sulfuric acid 1M, tetrabutylammonium hexafluorophosphate(TBAPF6) in acetonitrile (0.1M), and tetraethylammoniumtetrafluoroborate in propylene carbonate (1M) were used as the aqueous,organic, and ionic liquid electrolytes, respectively. Many other ionicliquids may be used as the electrolyte. The electrodes were rinsed witha solvent and dried after each measurement. Sulfuric acid showed thebest performance from a charging rate point of view.

TABLE 1 Comparison of effect of electrolyte on performance of the bareNb NWs yarns. Two scan rates of 50 mV/s and 500 mV/s scanning from −1 Vto 1 V were chosen for this comparison. Ionic Aqueous Organic LiquidC₅₀₀ (mF) 10 8.2 6.4 C₅₀ (mF) 10 8.8 10

The highest capacitances and power densities as shown in Table 1, wereachieved in sulfuric acid solution. The experimental values weremeasured to be 1.1×10⁷ F·m⁻³ (11 F·cm⁻³) and 1.3 kF·kg⁻¹ (1.3 F·g⁻¹)(for yarns made of nanowires with individual average diameter of 90 nm),which are close to the estimated theoretical limits, are higher than the0.5×10⁷ F·m⁻³ (5 F·cm⁻³) for carbon nanotube yarns, and close to thevalue of 1.2×10⁷ F·m⁻³ (12 F·cm⁻³) reported for densely packedsingle-walled carbon nanotubes in organic electrolyte (i.e.,Et₄NBF4/propylene carbonate). Using ionic liquids allows for operatingvoltages of up to 3 V. Capacitors of up to 36 mF can be made with bareniobium nanowires, which is higher than the largest capacitance value of10 mF made with PEDOT coated carbon nanotube yarns plied with Ptmicrowire. Although nanotubes have much smaller diameters, theircapacitance per area is much smaller, and there is a tendency forbundling to occur, reducing accessible surface area. For example, amongcarbon-based materials, graphene has the largest double layercapacitance of 0.21 F·m⁻² (21 μF·cm⁻²) but with total capacitance of0.065 F·m⁻² (6.5 μF·cm⁻²), which is almost 10 times lower than that ofniobium. This is explained by the fact that although the ion-accessiblearea is very high for graphene, the quantum capacitance—arising from thelow density of states—is small. In various embodiments, the niobiumnanowires generally do not suffer from this limitation in part due totheir relatively large diameter (about 90-140 nm), and capacitance isthus determined by the double layer.

Conductivity of Nb NW yarns was measured to be 3×10⁶ S·m⁻¹ which is 100times higher than in multiwalled carbon nanotube yarns (3×10⁴ S·m⁻¹). NbNW yarns were infiltrated with electrodeposited PEDOT, as shown in FIG.1C. Base Nb NW are depicted in FIG. 5A-F. For a 54 wt. % sample thevolumetric capacitance was improved 70 fold as depicted in FIG. 1D, upto 5×10⁷ F·m⁻³ (50 F·cm⁻³). This is explained with reference to FIG. 2,which schematically depicts a cell diagram and a charge storagemechanism of the half-cell. An SEM image of a piece of Nb NW yarn(slightly twisted) represents the working electrode and the separator isshown on the left. A double layer forms at the surface of the yarn veryquickly after voltage is applied. The charge then propagates to theinternal surface of the yarn. Coating the yarn with PEDOT (as depictedby the magnified circle) increases the charge storage due to the highcharge storage density of this conducting polymer, with the backbone ofthe polymer balancing ion charge. During charge storing, potential dropslinearly from the metal/electrolyte interface up to x₁ due to the highdensity of opposite charges accumulated at the interface. Ions alsocharge the interior surfaces of the niobium yarn. The inset illustratesthat the addition of PEDOT to the interior of the nanowire structure canincrease capacitance. Electrolyte and PEDOT within the yarn representpositive and negative ions, respectively.

Gravimetric capacitance increased generally linearly with PEDOT massfraction and directly correlated to the deposition time. In particular,with reference to FIG. 5 PEDOT was coated on yarns and mass fraction wasmeasured. As FIG. 5 illustrates, the gravimetric capacitance increasesalmost linearly with the mass fraction. This can be explained by thefollowing mathematical relationship between the PEDOT mass fraction (γ)and the specific capacitance.SC=(SC_(PEDOT)−SC_(Nb))×γ+SC_(Nb)

An advantage of metal nanowire capacitors is that they are much lessreliant on having a separate metal backing. Niobium is more conductivethan carbon nanotubes but less conductive than gold, so the addition ofgold as a backing was used to test if a rate of response could beincreased by reducing electrode resistance or possibly PEDOT:Nb contactresistance. Two pieces of 25 μm gold wire were wound around two PEDOTcoated niobium electrodes (65 mm active length and 100 μm diameter witha 9 μm thick separator in between) to act as charge collectors. Withreference to FIG. 6, a comparison of cyclic voltammograms of twoelectrode capacitors with and without the gold backing indicates thatthe resistance is lowered by the gold. The curves “a” and “b” are the CVcurves obtained before and after wrapping an Nb NW yarn sample with goldwires (at 100 mV/s). CV of the sample before PEDOT deposition are shownby curve “c”. This drop in resistance is a result of the lowerresistance along the length of the capacitor. Adding the gold can reducethe resistance, as determined by the slopes at the inflection points ofthe CVs in FIG. 6, from about 7Ω, down to just of 2Ω, which is theseparator resistance. The resistance of the niobium is then stilllimiting for long devices—but a carbon nanotube yarn based device wouldhave to be about 100 times shorter to achieve the same resistance, and10 times shorter for the same time constant. FIG. 7A shows thevolumetric capacitance (with unit of MF·m⁻³ or F·cm⁻³) of bare Nb NWyarn (with individual nanowire average diameter of 140 nm) as a functionof scan rate for a capacitor with a diameter of 85 μm per electrode anda separator thickness of 9 μm. FIG. 7B shows scaling of current (at zeropotential point) as a function of scan rate. At 20 V·s⁻¹ the current nolonger increases in direct proportion to scan rate. FIG. 7C showsconstant current charging and discharging of the supercapacitor before(at 0.3 A·g⁻¹, 1 A·g⁻¹, 2 A·g⁻¹, and 4 A·g⁻¹ from right to leftrespectively) and after (at 0.9 A·g⁻¹, 1.5 A·g⁻¹, 3 A·g⁻¹, and 3.7 A·g⁻¹from right to left respectively) depositing PEDOT (all per mass of dryelectrode). FIG. 7D is a Nyquist plot of the PEDOT coatedsupercapacitor, and the inset shows the Nyquist plots for bare and PEDOTcoated samples. FIG. 7E is a Bode plot of the supercapacitor before(solid line) and after (dashed-line) PEDOT deposition.

Constant current charge/discharge response of a Nb NW yarn before andafter coating with PEDOT is shown in FIG. 7C. The charge/discharge timefor the PEDOT coated sample increases, but it is nevertheless acapacitive response.

The combination of high conductivity of the metal nanowires and highvolumetric capacitance of the filler, i.e., pseudocapacitive materialsuch as PEDOT provides an opportunity to achieve both high energy andhigh power densities. Various separators (such as glass fibers,perfluorosulfonic acid polymer (such as Nafion™) membrane, and milliporemembrane) were tested. Cellulosic-based thin sheets (made of micronsized cellulosic wood pulp fibers) had the highest ionic conductivity(3.4 S·m⁻¹ in 1M sulfuric acid) with electrolyte uptake of up to 600%.In particular, different separator sheets were prepared by usingcellulosic wood pulp fiber of different sizes. The sizes are selectedsuch that the fibers can be classified as microfibers (MF). Theproperties of the separators are included in Table 2. The thickness ofthe separator sheets was measured (e.g., using an L&W Micrometer at 1 μmresolution). The electrolyte uptake of the separator sheets was obtainedby measuring the weight of dry sheets and then immersing the sheets in1M sulfuric acid for 2 hours. Finally, the sheets were removed fromelectrolyte and wiped with filter paper and the weight of the wet sheetswas recorded. The electrolyte uptake was then calculated by(m_(wet)−m_(dry))/m_(dry). The tensile strength of the sheets can beobtained using a tensile tester such as a QC-II tensile tester. Todetermine the wet strength of the sheet in electrolyte solutions, thesheets were cut into 7 mm stripes and kept in 1M sulfuric acid solutionfor 2 hours prior to tensile testing.

The surface morphologies of three different MF separators made withcellulosic wood fibers of 977 μm, 399 μm, and 177 μm are shown in FIGS.8A-8C. To clearly see the structure of microfibers, the samples werefreeze dried for SEM imaging to prevent collapse or shrinkage ofcellulosic fiber. The 9 μm thick separator made from 177 μm fibersshowed the best performance for aqueous electrolytes.

TABLE 2 Mechanical properties of some of the separators used in thedesign of the supercapacitor. Fiber Size Wet in Pulp Areal TensileTensile Suspension Thickness of Density Electrolyte Strength strength(μm) Sheet (μm) (g/m²) uptake (%) (Nm/g) (Nm/g) 977 179 60 209 53.5 0.9561 60 58 151 104.7 1.1 339 11 12 264 36.6 2.7 177 9 10 602 70.9 6.37Kim 104 20 200 4.9 0.71 wipes

Ionic conductivity of the separator plays an important role in the ESR.To measure the ionic conductivity of the separator, two flat sheets ofniobium foil (e.g., Alpha Aesar) were used with a cellulosic-based thinsheet separator in-between. The sheet was soaked in 1M sulfuric acid for30 min before performing the test. A few drops of 1M sulfuric acid wereadded to the separator after assembling the device. Ionic conductivityof 3.4 S m⁻¹ was measured for the 9 μm thick sample which corresponds tolower resistance (˜0.5Ω), than the total ESR of the supercapacitor (2Ω).The ionic resistance of the separator was also measured with a fourpoint ionic conductance apparatus. The results showed the same ionicconductivity value for the separator within experimental uncertainty.Ionic conductivity of separators made with perfluorosulfonic acidpolymer such as Nafion™, glass fibers, and millipore membranes were 0.06S m⁻¹, 0.58 S m⁻¹, and 0.08 S m⁻¹ respectively.

Having a highly ionically conductive separator can reduce the equivalentseries resistance, which can increase the power density. Due to thehydrophilicity and high tensile strength of microfiber cellulosefilm-based materials, they were used in various high power storagedevices described herein.

Wrapping gold or platinum micro wires around the infiltrated yarn ascharge collectors can improve the power density of supercapacitors madewith CNT yarns coated with conducting polymers. One may avoid usingexternal charge collector wires in the niobium device-performanceevaluations since the Nb nanowire yarns already have a highconductivity. Due to the high conductivity of the electrolyte, theseparator, and the electrode itself, Nb NWs supercapacitors may providehigh performance at fast charging rates with relatively little loss ofcapacitance as scan rate is increased to 50 V·s⁻¹, as shown in FIGS. 7Aand 7B.

To evaluate the scalability of a device, volumetric capacitance as afunction of diameter and inserted twist was measured. In particular,capacitance of bare niobium nanowire yarns was measured by dipping yarnsof same length and twist, but different diameters, in 1M sulfuric acidand performing cyclic voltammetry at scan rate of 50 mV/s. For thecounter electrode, one very large diameter Nb NW yarn was placed atabout 20 mm away from the working electrode. As FIG. 9 shows, volumetriccapacitance of the yarn appears to decrease somewhat as the diameterincreases. The yarn included a twist of 800 turns/m, and the volumetriccapacitance was measured at different diameters at a scan rate of 50mV/s. Since the scan rates are slow comparted to the charging time, thisapparent decrease may be due to the fact that as the diameter increasesfor the same twist, the density of the yarn increases, possibly reducingion accessible surface.

The capacitance of a 50 μm diameter Nb NW yarn, measured at 50 mV/s,decreased by almost 10% when a twist at 1700 turns/m was inserted, asshown in FIG. 10. Once again, this may be explained by the fact that asthe twist increases the porosity decreases; therefore, lession-accessible area may be available, likely leading to a decrease incapacitance. As the diameter increases by about 3.5 fold, the volumetriccapacitance only decreases by a factor of about 1.5, which is much lessthan the value reported for PEDOT coated CNT yarns, where the volumetriccapacitance can decrease by a factor of 2 corresponding to an increasein diameter by a factor of 1.4). Capacitance as a function of twistshows a less than 10% decrease in capacitance when the twist isincreased from zero twist to close to the breaking point, where thetwisted yarn may break.

The thread-like form of the capacitors suggests that they can beemployed in wound, knitted, braided, woven or knotted configurations.FIGS. 11A and 11B show flexible supercapacitor cells made with Nb NWsand PVA:H₂SO₄. FIG. 11A shows a 40 mm long twisted Nb NWs supercapacitorinfiltrated with solid electrolyte mounted on a glass slide. FIG. 11Bshows that the supercapacitor shown in FIG. 11A was cut from the tipinto several smaller cells to measure the relationship between lengthand capacitance of the cell. FIGS. 12A and B show CV curves of asupercapacitor (made from bare Nb NWs) before and after the performanceof 1000 bending tests, while FIG. 12C shows a CV curve for a device thatis knotted, as shown in FIG. 13A. In all cases the change in capacitancewas negligible. The knotted case shows that even when knotted there isno significant change in capacitance, after performing 1000 cycles ofeach deformation test. In the process of knotting a tight knot wasformed, with the radius of curvature approximately equal to twice theradius of the device (κ=0.4 mm⁻¹).

FIG. 13A shows the performance of a solid-electrolyte-basedsupercapacitor of length 50 mm and diameter 140 μm measured underdifferent deformation states for 1000 cycles. The unit for curvature is(mm)⁻¹. As shown in FIG. 13B, the sample was then bent at increasingangles and the performance was measured again. FIG. 13C shows the resultof a life cycle test of a sample in 2M sulfuric acid performed using 10mA for 20,000 cycles. FIG. 13D shows the first and last 5 cycles ofconstant current charge/discharge test for the 20 k life cycle test. Inthese experiments one cycle is considered to be one fullcharge/discharge from 0V to 1V to 0V. FIG. 13E shows the capacitance vslength relationship. One 40 mm long supercapacitor made with solidelectrolyte was shortened in 4 to 5 mm increments, and its capacitancewas measured at each step, to obtain the capacitance-lengthrelationship. These life cycle tests show that these devices canwithstand bending and knotting and, therefore, can be used in fabrics.

Electrochemical impedance spectroscopy was performed to measure the ESRand the frequency response of supercapacitors, as shown in FIGS. 7D and7E. An ESR of 2Ω was achieved in aqueous electrolyte (1M sulfuric acid).The electronic resistance of the 60 mm long metal nanowires is of asimilar magnitude suggesting that this, perhaps combined with theseparator resistance, is determining the ESR. A carbon multi-wallednanotube yarn of the same dimensions without a collector would likelyhave a resistance about 100 times larger, a similar capacitance, andthus about 100 times slower charging speed. The absence of an additionalcharge collection layer is an important advantage of using ananostructured metal electrode, rather than relatively poorly conductivecarbon, and reduces the contact resistance. For the embodiments having asolid electrolyte, the ESR is twice the liquid based supercapacitor. Thedominant time constants are 30 ms for the metal nanowire and 200 ms whenit is PEDOT infiltrated. An impedance model for the supercapacitor wasdetermined as follows.

Initial parameters for the electrochemical impedance modeling wereobtained from the Nyquist and Bode plots of the frequency response of anembodiment of a supercapacitor. A circuit model of the porous electrodein electrolytic media is shown in FIG. 14. R_(s) is solution resistance,C is the capacitance, R_(ct) is the charge transfer resistance, andZ_(D) is the diffusion impedance that can modelled with a Warburgelement or a transmission line. Warburg element with P=0.5 is used forthis modelling.

The model impedance was matched with the measured frequency responseshown in FIGS. 15A and 15B. FIG. 15A shows a Nyquist plot of a Nb/PEDOTsupercapacitor. The data points indicated by dots represent experimentalvalues and the curve line represents the simulation results from themodel. The subplot highlights the high frequency part of the Nyquistplot. FIG. 15B shows a Bode plot of the frequency response. The dashedlines represent the simulation results. Table 3 shows the time constantsand other parameters in the modelling.

TABLE 3 Circuit model parameters Parameters Symbol Value ESR R_(S) 1.86Ω Leakage resistance R_(L) 931 Ω Charge transfer resistance R_(ct) 1.29Ω Equivalent series inductance L_(s) 1.42 μH Double layer capacitanceC_(dl) 181 μF Capacitance C 74 mF

Along with the devices containing liquid electrolytes, solid electrolytesupercapacitors were made for flexible applications using PolyvinylAlcohol: sulfuric acid solid electrolyte. For devices of length 50 mmand diameter 140 μm, FIG. 16 shows Nyquist plots for niobium NWsupercapacitor before and after infiltrating with PVA as a solidelectrolyte. The subplot shows Nyquist plots in the higher frequencyregion. Electrochemical impedance spectroscopy (EIS) was performed from200 kHz to 22 mHz (for PVA:H₂SO₄) and from 200 kHz to 10 mHz (for bareNb NW yarns), at 6 points per decade. These results show that the ESRwas higher for PVA coated samples.

Ultimate tensile strength of niobium nanowire yarns, when twisted, canbe as high as 1.1 GPa which is higher than that of twisted carbonnanotube yarns and graphene yarns. This property combined with itsflexibility, which can be described in number of bends, knots, and/ortwists per unit length of the yarn, facilitates integration of Nb NWyarns into fabrics. To test the ability of the devices to bend, twistand knot, a solid electrolyte based supercapacitor was made. Theperformance was measured at different deformation states and the resultsshown in FIG. 13A indicate almost no change in the performance withbending and knotting. Bending tests were also performed to evaluateflexibility of the device and these tests show almost no changes incapacitance when bending from 0 to 180 degrees, as depicted in FIG. 13B.

Life cycle was measured by using a constant current charge/dischargetechnique. The capacitance was almost fully retained over 20,000 cycles,with the capacitance as a function of cycle number shown in FIG. 13C.Coulombic (or equivalently Faradaic) efficiency is close to unitythrough the 20,000 cycles performed. In particular, to determine theevolution of parasitic reactions over time, Faradaic (also known asCoulombic) efficiency of the supercapacitor was calculated from the datafor constant current charge/discharge at 10 mA (1 A/g) in 2M acidsulfuric for 20,000 cycles using the following equation:

$\eta_{c} = {\frac{Q_{out}}{Q_{i\; n}} = {\frac{t_{discharge}}{t_{charge}}.}}$

As FIG. 17 shows, this efficiency was steady around unity, whichindicates the electrochemical stability of the yarns and the separatorin acidic environment. FIG. 13D shows the first and last ten cycles ofconstant current charge/discharge almost completely match, therebyindicating no loss of performance at the end of the life cycle test.

One solid electrolyte-based supercapacitor was cut from the end and thecapacitance was measured, showing a linear relationship between lengthand capacitance, as shown in FIG. 13E. Accordingly, this indicates thatthe supercapacitor structure may contribute to the capacitance uniformlyalong the length.

FIG. 18 shows a comparison of Ragone plots, describing volumetric energyand power densities of various devices. Specifically, Curve A is aRagone plot for a 4V/500 μAh Li thin-film battery, Curve B is a plot fora MnO₂ carbon fiber supercapacitor, Curve C is a plot for a 2.75 V/44 mFactivated carbon electrochemical capacitor, Curve D is a plot for a 3.5V/25 mF supercapacitor, Curve E is a plot for a 2.7 V/1 F Maxwellsupercapacitor, Curve F is a plot for a 63V/220 μF electrolyticcapacitor, Curve G is a plot for a Panasonic (17 500) Li-ion battery,Curve H is a plot for a bare Nb NW yarn supercapacitor in ionic liquid(tetraethylammonium tetrafluoroborate in propylene carbonate (1M))operating at 3V, Curve I is a plot for a PEDOT coated Nb NW yarnsupercapacitor, and Curve J is a plot for a two-ply CNT/PEDOT electrodesupercapacitor.

Peak power density and energy density of 55 MW·m⁻³ (55 W·cm⁻³) and 25MJ·m⁻³ (7 mWh·cm⁻³) were measured for a supercapacitor made from twobare niobium nanowire electrodes with a separator, which are higher thanthose measured for ultrafast charging supercapacitors withCNT/PEDOT/Gold, as shown in FIG. 18. However, the average power density,as shown by the curves ‘H’ ad ‘I’ in FIG. 18, is lower than the averagepower density of the CNT/PEDOT/Gold. CV curves scanning from 0V to 3Vfor the bare niobium yarns and PEDOT coated Nb NW yarns that aredepicted in FIGS. 19 and 20, respectively were used to calculate thevarious data points used in the Ragone plots shown in FIG. 18. FIG. 19shows CV curves for a cell with ionic liquid for scan rates. Curves“a,”, “b,” “c,” and “d” correspond to scan rates of 100 mV·s⁻¹, 1,000mV·s⁻¹, 10,000 mV·s⁻¹, and 40,000 mV·s⁻¹, respectively. The electrodeswere each 60 mm long with diameter of 75 μm. FIG. 20 shows CV curves fora PEDOT coated cell with ionic liquid for various scan rates. Curves“a,” “b,” “c,” and “d” correspond to scan rates 100 mV·s⁻¹, 200 mV·s⁻¹,500 mV·s⁻¹, and 1,000 mV·s⁻¹, respectively. The electrodes were each 97μm thick, 508 μm wide, and 45 mm long. The high capacitance and energydensities are the result of the high surface capacitance of niobiumrelative to carbon, and can compensate for the relatively large typicaldiameter of the niobium nanowire, which is 140 nm, compared to 10 nm ofcarbon multi-walled nanotubes.

The niobium nanowires may be extracted from copper-niobium compositewires as described in Mirvakili, S. M.; Pazukha, A.; Sikkema, W.;Sinclair, C. W.; Spinks, G. M.; Baughman, R. H.; Madden, J. D. W.“Niobium Nanowire Yarns and their Application as Artificial Muscles,”Adv. Funct. Mater. 2013, 23, 4311-4316, the entire contents of which areincorporated herein by reference. Nanowires of a transition metal AND OFALUMINUM? can also be extracted using techniques described in U.S. Pat.No. 5,088,183, to Kanithi, entitled “Process for Producing Fine andUltrafine Filament Superconductor Wire,” the entire contents of whichare incorporated herein by reference. Two different techniques may beused to etch the Cu—Nb matrices. Chemical etching is performed by usingsolution of Nitric acid and de-ionized water. Percentage of the solventcan vary depending on thickness of the raw Cu—Nb matrix. In a secondtechnique, electrochemical reactions may be used to remove the copper.Samples can be immersed in copperic sulfate solution and connected tothe positive node of a power supply. A piece of pure copper can beconnected to the other polarity and may be immersed in the solution, aswell. By applying voltage, copper can be etched away from Cu—Nb sample.Two electrodes may then made from the resulting niobium nanowires byadding small amount of twist to each yarn.

Two 10 mm by 50 mm sheets of niobium thin foil were used with 1Msulfuric acid to measure the capacitance per area of niobium. Electricalconductivity of Nb NW yarns may be measured by using 4-point probetechnique as described in Mirvakili, S. M.; Pazukha, A.; Sikkema, W.;Sinclair, C. W.; Spinks, G. M.; Baughman, R. H.; Madden, J. D. W.“Niobium Nanowire Yarns and their Application as Artificial Muscles,”Adv. Funct. Mater. 2013, 23, 4311-4316.

Volumetric and gravimetric capacitance, power density and energy densityof various embodiments of Nb NW-based supercapacitors were calculated asfollows. The volumetric and gravimetric capacitance of the bare NbNW-based supercapacitors were calculated by using

${C_{m} = \frac{2C_{tot}}{m}},{{{and}\mspace{14mu} C_{V}} = {C_{m} \times \rho_{Nb}}},$where m is mass of one electrode for the cases of using similarelectrodes and ρ_(Nb) is the density of bulk niobium.

The total capacitance was measured using cyclic voltammetry and thenusing C_(tot)=I×ν⁻¹ from the CV curves, where ν is the scan rate and Iis the value of the current at the symmetry axis, which was at potentialof 0 V unless stated otherwise. For PEDOT coated samples the volume ofthe one electrode was measured and used to find the C_(V). Manyembodiments with PEDOT coated Nb NWs were symmetrical i.e., the twoelectrodes are similar. For bare Nb NW supercapacitors, in someembodiments, the two electrodes had different masses. As such, thefollowing relation was used to find the capacitance of each electrode:

${C_{1} = {C_{tot}\left( {1 + \gamma} \right)}},{C_{2} = \frac{C_{tot}\left( {1 + \gamma} \right)}{\gamma}},$where γ is the mass ratio (m₁/m₂).

For life cycle measurements, capacitance was found from the slope forthe constant current charge/discharge curves using

$C_{tot} = {\frac{I}{{dV}/{dt}}.}$

For generating the Ragone plot, volumetric power density (P_(V(av))(W·m⁻³)) and energy density (E_(V) (J·m⁻³)) (at device level (V_(tot)))were calculated at each scan rate of ν (V·s⁻¹) by integrating under thecyclic voltammetry curves (during the discharge cycles—initial potentialat E_(i)) as follows:

${P_{V{({av})}} = {\frac{1}{V_{tot}E_{i}}{\int_{E_{i}}^{0}{IEdE}}}},{E_{V} = {\frac{1}{V_{tot}v}{\int_{E_{i}}^{0}{{IEdE}.}}}}$

The following equations may be used to calculate the peak volumetricpower and energy density.

${E_{V} = \frac{C_{tot}E^{2}}{2V_{tot}}},{P_{V} = {\frac{E^{2}}{4R_{ESR}V_{tot}}.}}$

EIS was performed by sweeping one sinusoidal frequency with amplitude of20 mV and DC bias of 0.2 V from 200 kHz to 10 mHz. PVA with sulfuricacid was used for the solid electrolyte based supercapacitors. Sulfuricacid, deionized water, and PVA were mixed with mass ration of 1:10:1 andstirred aggressively for 1 hour at 90° C.

The transition-metal-based yarns such as Nb NW yarns generally showhigher capacitance and energy per volume, are stronger, and are 100times more conductive than similarly spun carbon multi-walled nanotube(MWNT) and graphene yarns. The long niobium nanowires, formed byrepeated extrusion/drawing can achieve device volumetric peak power andenergy densities of 55 MW·m⁻³ (55 W·cm⁻³) and 25 MJ·m⁻³ (7 mWh·cm⁻³), 2and 5 times higher than for state-of-the-art CNT yarns, respectively.The capacitance per volume of Nb nanowire yarn is lower than the 158MF·m⁻³ (158 F·cm⁻³) reported for carbon-based materials such as reducedgraphene oxide (RGO)/CNT wet-spun yarns, but the peak power and energydensities are 200 and 2 times higher. Achieving high power in long yarnsis made possible by the high conductivity of the metal, while highenergy density is possible at least in part due to the high internalsurface area. In some embodiments, no additional metal backing isneeded, unlike for CNT yarns and supercapacitors in general, savingsubstantial space. By infiltrating the yarn with pseudo-capacitivematerials such as PEDOT the energy density can be further increased to10 MJ·m⁻³ (2.8 mWh·cm⁻³). Niobium nanowire yarns are generally highlyflexible and, as such, can be woven into textiles and use in wearabledevices.

High performance energy storage devices made as described above fromnanowires of transition metals such as niobium are flexible and sewable.The capacitance of these devices can be controlled by selecting thelength of the yarn. In general, three mechanism of charge storage canoccur: charge storage in the electric double layer at thenanowires/electrolyte interface (i.e., non-Faradaic storage), redoxreaction of electrolyte with nanowires (i.e., Faradaic storage), andpseudo-capacitance, by coating the nanowires with other materials ofhigh specific capacitance (i.e., Faradaic storage).

With reference to FIG. 21, an Nb NW based supercapacitor structure 2100includes a pair of niobium yarns 2102, 2104. Each yarn can be made usinga number of niobium nanowires. The diameter of each individual niobiumnanowire can be selected from a range 20 nm to 200 nm. The diameter ofeach yarn 2102, 2104 may be selected from a range of 10 μm up to 1 mm.In some embodiments of the supercapacitor 2100, the diameter of bothyarns 2102, 2104 is the same. In other embodiments, however, thediameters of the two yarns can be different. The length of each yarn canbe selected from a range of 10 nm up to 10 m. For example, the lengthsof the two yarns 2102, 2104 can be 10 nm, 15 nm, 50 nm, 200 nm, 1 μm, 10μm, 150 μm, 500 μm, 1 mm, 2 mm, 6 mm, 2 cm, 25 cm, 60 cm, 1 m, 2.4 m, 8m, 10 m, 30 m, 100 m, etc.

Each yarn 2102, 2104 is infiltrated with a flexible, solid electrolyte.One example of a flexible, solid electrolyte is a combination of PVA andsulfuric acid, prepared as described above. A solid electrolyte can alsobe formed by combining an ionic liquid and fine inert nanoparticles,such as fused silica nanoparticles. In general, a suitable solidelectrolyte has ions and ionic conductivity. The yarns 2102, 2104 may betwisted around each other. In some embodiments, a single Nb NW yarn maybe looped as shown in FIG. 13E, and the two strings of the loop may betwisted together. The loop may then be cut to separate the two stringsinto two yarns of the supercapacitor.

Two electrically conductive pads 2106, 2108 may then be attached to thetwo yarns 2102, 2104, respectively, forming two electrodes, namely,anode and cathode, of the supercapacitor 2100. The pads can be formedusing a highly conductive metal such as gold, silver, copper, etc., orthe pads can be thin niobium plates. A pad can be a piece of thin foilclamped to the yarn. A pad may also be attached or formed byelectrochemical plating. Leads 2110, 2112 may be attached to the twopads 2016, 2108, respectively, for electrically connecting thesupercapacitor 2100 with components of a circuitry.

The two yarns 2102, 2104 may be optionally infiltrated withpseudocapacitive materials such as conductive polymers, MnO₂, RuO₂,etc., so as to increase the volumetric capacitance of the supercapacitor2100. Alternatively, or in addition, the yarns 2102, 2104 may be coatedwith activated carbon or graphene to increase the surface area forcharge collection. Examples of suitable conductive polymers includepoly(3,4-ethylenedioxythiophene) (PEDOT), poly pyrrole, and polyaniline. The conductive polymer can be electrodeposited on each yarns2102, 2104. In one embodiment, the volumetric capacitance of asupercapacitor having yarns infiltrated with PEDOT was about 50 F/cm³,which is an approximately 70 times improvement over the volumetriccapacitance of a supercapacitor made with bare Nb NW yarns.

In some embodiments, a long pair (e.g., 10 cm, 25 cm, 1.5 m, 3 m, etc.,and, in general, up to 100 m long pair) of two yarns that are twistedtogether may be cut into two or more pieces. Using the generally linearrelationship between the capacitance of the pair and the length thereof,as illustrated above with reference to FIG. 13E, the lengths of one ormore cut portions can be selected according to a specified capacitance.A pair of electrode pads may then be attached to each cut pair of yarnsto form two or more supercapacitors, one or more of which have arespective specified capacitance. The capacitance of a supercapacitorcan thus be tuned by cutting a twisted pair yarn according to a lengththereof as determined by the desired capacitance.

With reference to FIG. 22, each of the two Nb NW yarns 2202, 2204 isencased within niobium foils 2203, 2205. The yarns and the foils areplaced within an enclosure 2206 forming a supercapacitor 2200. Theindividual niobium nanowires of a particular yarn may be twistedtogether, or may be disposed together as shown. The diameters of the twoyarns can be the same or they can be different. In general, the diameterof each yarn 2202, 2204 can be selected from a range 10 μm to 1 mm. Thelength of each yarn can be selected from a range 1 μm to 100 m.Nanowires having a diameter in the range 20 nm to 200 nm can be selectedto form the yarns 2202, 2204.

The enclosure 2206 can be metallic or non-metallic such as plastic,ceramic, etc. The enclosure may include a separator 2208 that isdisposed between the two yarns 2202, 2204. In addition, the enclosuremay be filled with a liquid electrolyte 2210. The separator 2208preferably includes an ionically conductive material. In variousembodiments, the separator 2208 may include one or more of: glassfibers, perfluorosulfonic acid polymer such as Nafion™, one or moremillipore membranes, and one or more cellulosic-based sheets. Thecellulosic-based sheet(s), constructed as described above, may includemicron-sized cellulosic wood pulp fibers. The thickness of the separator2208 may can be selected from a range 1 μm to 100 μm, e.g., about 9 μm.The liquid electrolyte 2210 can be an aqueous electrolyte (e.g.,sulfuric acid), an organic electrolyte (e.g., tetrabutylammoniumhexafluorophosphate (TBAPF6) in acetonitrile), or an ionic electrolyte(e.g., tetraethylammonium tetrafluoroborate in propylene carbonate). Amolten salt can be used as a liquid electrolyte, e.g., when thesupercapacitor is to be operated at high temperatures, e.g., up to about1200° C.

Two electrically conductive pads 2212, 2214 may be attached to the twofoils 2203, 2205, forming an anode and a cathode, respectively, of thesupercapacitor 2200. Alternatively, the conductive pads may be attachedto each yarn 2202, 2204. The pads can be formed using a highlyconductive metal such as gold, silver, copper, etc., or the pads can bethin niobium plates. Leads 2216, 2218 can be used to electricallyconnect the supecapacitor 2200 with one or more components of acircuitry.

In one embodiment, described with reference to FIGS. 23A and 23B, a pairof yarns 2302 a, 2302 b of nanowires including aluminum or a transitionmetal such as niobium, tantalum, vanadium, molybdenum, copper, nickel,iron, platinum, gold, silver, and zinc, and a pair of separators 2304 a,2304 b are alternately disposed. The yarns 2302 a, 2302 b can beoptionally infiltrated with a pseudocapacitative material such as aliquid polymer. The separators can be made using one or more of: glassfibers, perfluorosulfonic acid polymer such as Nafion™, one or moremillipore membranes, and one or more cellulosic-based sheets. A pair ofmetallic wires 2306 a, 2306 b, e.g., wires of gold, silver, platinum,copper, aluminum, or the transition metal used to make the yarn, areclamped around the yarns 2302 a, 2302 b, respectively. To this end, theends of the wires 2306 a, 2306 b may be flatted, e.g., by rolling, toform heads 2308 a, 2308 b that can be wrapped around the respectiveyarns 2302 a, 2302 b. The structure thus formed is then rolled around asdepicted in FIGS. 23C and 23D to form a supercapacitor 2300.

Each of the separators 2304 a, 2304 b can include one or morecellulosic-based thin sheets, which may also function as electrolyteabsorbers. To make such separators, micron sized cellulosic wood pulpfibers are obtained by refining softwood pulp. Different fiber sizes maybe collected at different refining energies. The average fiber size in apulp suspension can be measured (e.g. using a Scircco Malvern 2000Mastersizer). In general, separator sheets can be prepared using pulpshaving different fiber sizes, e.g., 977, 560, 340, and 177 μm. It shouldbe understood that other sizes, and fewer (e.g., only one fiber size) ormore than four fiber sizes can also be used in making the separators. Tomake the separators 2304 a, 2304 b sheets with fiber size of about 977μm, one may use, e.g., a handsheet former. The fiber suspensions withfiber sizes of 560, 340, and 177 μm may be diluted to 0.2 wt %consistency in distilled water followed by stirring at 1000 rpm for 15min. The suspensions may be poured into petri dishes after 10 min vacuumdeaeration and dried at room temperature.

Poly (3,4-ethylenedioxythiophene) (PEDOT) was deposited on the yarns2302 a, 2302 b using a galvanostatic deposition technique with currentdensity of 0.8 A·m⁻² at room temperature. The deposition solution wasprepared by making a 0.1M tetrabutylammonium hexafluorophosphate(TBAPF6) solution in 99% propylene carbonate and 1% water later mixedwith 0.1M EDOT. In some embodiments, the yarns are not coated with PEDOTbut may be coated with a different liquid polymer such as poly pyrroleor poly aniline. In some embodiments, one or both yarns are not coatedwith any pseudocapacitative material or a liquid polymer.

In another embodiment, with reference to FIG. 24, a supercapacitor 2400includes a metallic electrode 2402 a. The electrode 2402 a can be a thinplate or a foil of gold, silver, copper, aluminum, platinum, etc.,having a thickness selected from the range 10 μm to 50 μm. The area ofthe metallic electrode 2402 a may be selected from a range 6 mm² to 600mm². Nanowires 2404 a including aluminum or a transition metal such asniobium, tantalum, vanadium, molybdenum, copper, nickel, iron, platinum,gold, silver, and zinc may be attached to the metallic electrode 2402 a.Preferably, the nanowires 2404 a are at an angle in a range from 70° to110° relative to the surface of the metallic electrode 2402 a. Thediameter of the nanowires 2404 a is selected from a range 20 nm to 200nm, and the average length of the nanowires 2404 a is selected from arange 1 μm to 1000 μm.

The volumetric space of the electrode 2402 a can be specified as theproduct of the area of the electrode 2402 a and the average length ofthe nanowires 2404 a. In some embodiments, the nanowires are depositedon the electrode such that only up to about 20% of the volumetric spaceof the electrode 2402 a is consumed by the nanowires 2404 a. Theavailable free space can be used to coat the nanowires with othermaterials, such as liquid polymers. This can increase the capacitance ofa supercapacitor formed using the electrode 2402 a having the nanowires2404 a attached thereto.

A second electrode 2402 b, having nanowires 2404 b attached thereto maybe similarly formed as the first electrode 2402 a. The two electrodesmay be disposed such that the free ends of the nanowires 2404 a point tothe electrode 2402 b and the free ends of the nanowires 2404 b point tothe electrode 2402 a, forming a supercapacitor 2400. A separator 2406 isdisposed between the two groups of nanowires 2404 a, 2404 b. Theseparator 2406 can be made using one or more of: glass fibers,perfluorosulfonic acid polymer such as Nafion™, one or more milliporemembranes, and one or more cellulosic-based sheets. The distance betweenthe two electrodes 2402 a, 2402 b is generally determined by thethickness of a separator disposed there between. Typically, thethickness of the separator can be selected from a range 1 μm to 100 μm.

Optionally, in some embodiments, the structure including the twoelectrodes 2402 a, 2402 b, the two groups of nanowires 2404 a, 2404 b,and the separator 2406 can be disposed within a sealed enclosure filledwith an ionically conductive liquid electrolyte, examples of which areprovided above. In these embodiments, the metal used to form theelectrodes and the electrolyte are selected to be chemically compatiblewith each other, and/or to have low interfacial polarization.

In one embodiment, the structure that includes the electrode 2402 a,having nanowires 2404 a attached thereto is fabricated as follows. ANb—Cu nanocomposite wire is cut using an electro-discharge machine (EDM)into pieces that are 250 μm long. In other embodiments, the length ofthe pieces can be any number between 1 μm and 1,000 μm, such as 2, 10,50, 100, 120, 200, 500, 600, or 725 μm. The length of the pieces can beselected according to a desired capacitance. The cut pieces are groupedtogether into a rectangular shape of size of 3 mm×4 mm. Other sizes(e.g., 1 mm×1 mm, 2 mm diameter, 50 mm², etc.), and shapes, such ascircular, oval, square, etc., are also possible. The grouped pieces maythen be polished with sand paper in two steps. The first step involvesusing a coarse sand paper to remove any residues from the EDM processand the second step involves using a fine sand paper to make the top andbottom surface of the pieces clean and smooth.

Next, the grouped pieces may be rinsed with acetone, ethanol, anddeionized (DI) water. After rinsing they are sonicated in a 3:2 ratiosolution of (50% nitric Acid and DI water):(DI water) for 1 hour.Thereafter, one side of each grouped piece is coated with hot glue orany dissolvable adhesive to protect that surface from beingelectroplated. The grouped pieces are then electroplated with gold. Toensure deposition of gold over the niobium nanowires the selectedcurrent density is 0.34 mA/m², and the selected duration ofelectroplating is 18 hours. After performing the electroplating thegrouped nanowire pieces are rinsed with DI water and the maskingadhesive is removed. Thereafter, the electroplate nanowires are etchedwith a 50% nitric acid solution for 48 hours. After etching the matrixin nitric acid solution, gold platted niobium nanowire electrodes areformed.

In an embodiment, with reference to FIG. 25, a supercapacitor 2500includes niobium yarn 2502 made by twisting together several niobiumnanowires 2504 that are processed as described below. The number ofnanowires in a yarn generally depends on the average diameter of thenanowires and the diameter of the yarn. In general, the diameter of theyarn 2502 can be selected from a range 10 μm to 1 mm, and nanowireshaving a diameter in the range 20 nm to 200 nm can be used to form theyarn. The length of the yarn 2502 can be selected from a range 1 μm to100 m.

A portion of each nanowire 2504 is oxidized to form an outer surfacethereof a layer of niobium pentoxide 2506. The nanowire 2504 is theninfiltrated or coated with a conductive polymer such as PEDOT, polypyrrole, and/or poly aniline, forming a cathode 2508. The conductivepolymer can be electrodeposited on the nanowire 2504. Alternatively, thenanowire 2504, having a layer 2506 of niobium pentoxide, is coated witha liquid metal such as indium, gallium, or tin to form the cathode 2508.The nanowire 2504 forms the anode. In some embodiments, the anode 2504may be oxidized to form niobium oxide, and then, a portion of theoxidized niobium wire is further oxidized for form the niobiumdielectric layer 2504.

In various embodiments, the diameter of a nanowire 2504 and thickness ofniobium pentoxide can be selected according to a desired operatingvoltage of the supercapacitor 2500. In particular, the dielectricbreakdown of Nb pentoxide is approximately 400 V/μm. Therefore, for anoperating voltage of 40 V, the Nb pentoxide layer of a thickness ofapproximately 100 nm and a Nb nanowire of diameter of approximately 240nm are selected. Similarly, if the desired operating voltage isapproximately 20 V, a 50 nm thick Nb pentoxide layer may be formed. Foran operating voltage of 3 V, a 7.5 nm thick Nb pentoxide layer isneeded. The supercapacitors described above with reference to FIGS.21-24, that do not include a Nb pentoxide dielectric layer can beoperated at voltages up to 3V, e.g., at operating voltages 2.8V, 2.5V,2.2V, 1.8V, 1.5V, 1V, 0.5V, etc.

Various embodiments and electrical analysis thereof demonstrates thatniobium nanowire electrodes can achieve performance levels similar tocarbon nanotube and graphene-based devices. The high conductivity ofniobium, its good mechanical properties and, its high surfacecapacitance make this metal a viable alternative to carbons. The hightensile strength and better volumetric capacitance relative to CNT-basedsupercapacitors make Nb NW based supercapacitors particularly suitablefor use in wearable devices.

Due to the fast charging capabilities of the Nb NW based capacitors,they can be used in circuits where high current pulses are required. Fordemonstration purposes, one 36 mF supercapacitor was made with bare Nbnanowires (with ionic liquid electrolyte) and was used to store energyfrom a solar cell every 10 seconds and then release it to a 30 mW FMtransmitter with minimum operating voltage of 2.5 V. Due to theflexibility of the electrodes, these devices can be integrated intosystems without imposing significant design or dimension constraints.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyinventions or of what may be claimed, but rather as descriptions offeatures specific to particular embodiments of particular inventions.Certain features that are described in this specification in the contextof separate embodiments can also be implemented in combination in asingle embodiment. Conversely, various features that are described inthe context of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are described in a particular order, thisshould not be understood as requiring that such operations be performedin that order only or in a sequential manner, or that all describedoperations be performed, to achieve desirable results. In general,substantially, about, or approximately in connection with a parametermeans within a small percentage (e.g., within 0.2%, 0.5%, 1%, 2%, 5%,10%, etc.) of a specified value of the parameter.

Thus, particular embodiments of the subject matter have been described.Other embodiments are within the scope of the following claims. In somecases, the actions recited in the claims can be performed in a differentorder and still achieve desirable results.

What is claimed is:
 1. A supercapacitor comprising: a niobium yarncomprising a plurality of niobium nanowires, each niobium nanowirecomprising at least three sections, wherein: a first section comprisesat least one of unoxidized niobium and niobium oxide; a second sectioncomprises a niobium pentoxide layer; and a third section comprises alayer formed by coating the niobium nanowire in at least one of aconductive polymer and a liquid metal.
 2. The supercapacitor of claim 1,wherein the liquid metal comprises at least one of indium, gallium, andtin.
 3. The supercapacitor of claim 1, wherein: a diameter of theniobium nanowire is selected from a range of 20 nm to 200 nm; and athickness of the second section is selected from a range of 7.5 nm up to100 nm.
 4. The supercapacitor of claim 1, wherein a diameter of theniobium nanowire and a thickness of the second section are selectedaccording to a specified operating voltage.
 5. A method of constructinga supercapacitor, the method comprising the steps of: for each one of aplurality of niobium nanowires: oxidizing a portion of a niobiumnanowire, thereby forming a dielectric layer of niobium pentoxidedisposed on an anode portion of the niobium nanowire, the anode portioncomprising at least one of unoxidized niobium and niobium oxide; andcoating the niobium wire having the anode and the dielectric layer withat least one of a conductive polymer and a liquid metal, thereby forminga cathode; and forming a niobium yarn by grouping the niobium wirestogether, each wire having an anode, a dielectric layer, and a cathode,to form the supercapacitor comprising the niobium yarn.
 6. The method ofclaim 5, wherein the liquid metal comprises at least one of indium,gallium, and tin.
 7. The method of claim 5, wherein: (i) a diameter ofeach niobium nanowire is selected from a range of 20 nm to 250 nm; and(ii) oxidizing is controlled such that a thickness of the dielectriclayer is within a range of 7.5 nm up to 100 nm.
 8. The method of claim5, further comprising: selecting a diameter of each niobium nanowire anda thickness of each dielectric layer to facilitate operation of thesupercapacitor at a specified operating voltage.